feat: separate landing page from dashboard and add auth protection
- Move dashboard from / to /dashboard route
- Create new landing page at / for logged-out users
- Add auth redirects: landing → dashboard (logged in), dashboard → login (logged out)
- Protect calendar, following, and lists pages (redirect to login if not authenticated)
- Update Header navigation to point to /dashboard
- Fix redirect crash by using useNavigate hook instead of throw redirect